This section is intended to provide a background or context to the disclosed embodiments that are recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Watermarks are substantially imperceptible signals embedded into a host content. The host content may be any one of audio, still image, video or any other content that may be stored on a physical medium or transmitted or broadcast from one point to another. Watermarks are designed to carry auxiliary information without substantially affecting fidelity of the host content, or without interfering with normal usage of the host content. For this reason, watermarks are sometimes used to carry out covert communications, where the emphasis is on hiding the very presence of the hidden signals. Other widespread applications of watermarks include prevention of unauthorized usage (e.g., duplication, playing and dissemination) of copyrighted multi-media content, automatic content recognition (ACR), proof of ownership, authentication, tampering detection, content integrity verification, broadcast monitoring, transaction tracking, audience measurement, triggering of secondary activities such as interacting with software programs or hardware components, communicating auxiliary information about the content such as caption text, full title and artist name, or instructions on how to purchase the content. The above list of applications is not intended to be exhaustive, as many other present and future systems can benefit from co-channel transmission of main and auxiliary information.
In some applications of watermarks, such as content management applications, watermarks that are embedded in a host content must often be extracted in consumer products (e.g., DVD players, software media players, etc.) that access and use the content. These devices must interpret the information that is conveyed by such watermarks, and carry out the appropriate actions (e.g., enforcement actions) in conformance with the corresponding content use policies. Examples, of such actions include allowing full access to the content, preventing access to the content, allowing the playback of the content for a limited duration, allowing access to an abbreviated or degraded version of the content, etc. To provide a viable content management system, watermark extraction and subsequent operations in such widely available consumer products must be protected against potential attacks that are aimed at gaining unauthorized access to the content.